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Recent studies on Cu interconnects have shown that interface diffusion between Cu and the cap layer dominates mass transport for electromigration. The kinetics of mass transport by interface diffusion strongly depends on the material and processing of the cap layer. In this series of two papers, we report in Part I the interface and grain-boundary mass transport measured from isothermal stress relaxation in electroplated Cu thin films with and without a passivation layer and in Part II a kinetic model developed to analyze the stress relaxation based on the coupling of grain boundary and interface diffusion. We show that a set of isothermal stress relaxation experiments together with appropriate modeling analysis can be used to evaluate the kinetics of interface and grain-boundary diffusion that correlate to electromigration reliability of Cu interconnects. Thermal stresses in electroplated Cu films with and without passivation, subjected to thermal cycling and isothermal annealing at selected temperatures, were measured using a bending-beam technique. Thermal cycling experiments showed the effect of passivation and provided information to select the initial stresses and temperatures for isothermal stress measurements. Isothermal experiments at moderate temperatures showed a significant transient behavior of stress relaxation. Based on the kinetic model developed in Part II, grain boundary and interface diffusivities were deduced. While the deduced grain boundary diffusivity reasonably agrees with other studies, the diffusivity at the Cu/ SiN cap layer interface was found to be generally lower than the grain-boundary diffusivity at the temperature range of the present study.

Polycrystalline samples of perovskite-structured BaCeO, and SrCeO, were studied by Raman scattering spectroscopy in the temperature range 77-983 K. Changes in the spectra of BaCeO, with temperature are explained in terms of a second-order phase transition (DZh + D4,) at To = 427 K. Analysis of the temperature dependence of soft modes suggests a partial displacive character. A dramatic decrease in band intensity above 1O00 K indicates a transition to the cubic (0,) structure, for which no Raman bands are expected. The Raman spectra of SrCeO, indicate that no phase transitions occur in the temperature range 77-983 K. Room temperature spectra of BaCeO, doped with various rare earth ions (Nd3+, Gd3+, Yh3+) are compared. Doping with 5 and 10 mol.% Nd stabilizes the high-temperature tetragonal and cubic phases, respectively. The crystal structure is a sensitive function of Nd dopant concentration.

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